Lasers that emit light energy at wavelengths in the range of about 1,300 to about 11,000 nanometers (long wavelength or “LW” laser energy) are excellent vaporizers of tissue, since their energy is highly absorbed by water, a major constituent of mammalian tissue. When exposed to such long wavelength laser energy, the water in the tissue is rapidly heated and converted to steam, causing ablation or vaporization of the tissue. These properties make long wavelength lasers particularly useful for nonsurgical removal or reduction of tissue.
Typically, laser energy is delivered to a tissue site via an optical fiber or an optical wave guide device adapted for transmission of long wavelength laser energy. The emitting end of the fiber optic or wave guide is placed in close proximity to the desired tissue site. An endoscope is first positioned inside a duct, body cavity, hollow organ or surgically created passageway at the tissue site. The energy emitting end of the optical fiber or wave guide is then threaded through a channel in the endoscope to place the emitting end of the optical fiber or wave guide in the optical position near the tissue. Typically a fiber optic viewing device is also positioned at the working end of the endoscope to view the tissue site as it is being irradiated with laser light energy and to verify the correct positioning of the emitted laser energy.
However, when water, saline, or other aqueous liquid is infused through the endoscope to provide a clear field of view of the tissue inside a duct, body cavity, hollow organ, or surgically created passageway, via the fiber optical viewing device, a substantial amount of the laser energy is wasted. The aqueous liquid between the distal end of the optical fiber and the target tissue absorbs a substantial part of the light energy and creates a steam bubble, which acts as an “optical cavity.” The remainder of the laser energy passes through the steam bubble and vaporizes or ablates the target tissue. However, as the steam bubble collapses between pulses of laser energy, liquid flows back into the space between the distal end of the optical fiber and the target tissue, requiring some of the laser energy to again be consumed in recreating the steam bubble between the optical fiber and the tissue, diminishing the amount of laser energy reaching the target tissue.
The above phenomenon was first described by Jeffrey M. Isner et al. in “Mechanism of laser ablation in an absorbing fluid field,” Lasers Surg. Med. 1988;8(6):543-54, and is commonly referred to as the “Moses Effect” or “parting the water.”
Holmium lasers emitting long wavelength light were used in the mid to late 1990's to resect prostate tissue, as described above. However, while the procedure produced benefits comparable to a trans-urethral resection of the prostrate or “TURP” procedure, in which a wire loop is heated by radiofrequency (“RF”) energy to cut-out swaths of prostate tissue, the laser procedure typically took about 45 minutes to an hour for a small (20-30 gram prostate) and longer than an hour for larger prostates. As a result, the laser procedure never became popular and is presently used only infrequently.
It would be desirable to enable a substantially greater amount of light energy from long wavelength lasers to be used in an aqueous liquid environment to vaporize tissue without significant quantities of energy being wasted by vaporizing the intervening aqueous liquid.